package org.apache.lucene.index;

/**
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements. See the NOTICE file distributed with this
 * work for additional information regarding copyright ownership. The ASF
 * licenses this file to You under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 * 
 * http://www.apache.org/licenses/LICENSE-2.0
 * 
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 * License for the specific language governing permissions and limitations under
 * the License.
 */

import java.io.IOException;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;

import org.apache.lucene.document.Document;
import org.apache.lucene.document.Field;
import org.apache.lucene.document.Fieldable;
import org.apache.lucene.document.Field.Index;
import org.apache.lucene.document.Field.Store;
import org.apache.lucene.index.IndexWriterConfig.OpenMode;
import org.apache.lucene.store.Directory;
import org.apache.lucene.store.LockObtainFailedException;
import org.apache.lucene.util.Version;

/**
 * A {@link SnapshotDeletionPolicy} which adds a persistence layer so that
 * snapshots can be maintained across the life of an application. The snapshots
 * are persisted in a {@link Directory} and are committed as soon as
 * {@link #snapshot(String)} or {@link #release(String)} is called.
 * <p>
 * <b>NOTE:</b> this class receives a {@link Directory} to persist the data into
 * a Lucene index. It is highly recommended to use a dedicated directory (and on
 * stable storage as well) for persisting the snapshots' information, and not
 * reuse the content index directory, or otherwise conflicts and index
 * corruptions will occur.
 * <p>
 * <b>NOTE:</b> you should call {@link #close()} when you're done using this
 * class for safetyness (it will close the {@link IndexWriter} instance used).
 */
public class PersistentSnapshotDeletionPolicy extends SnapshotDeletionPolicy {

    // Used to validate that the given directory includes just one document w/ the
    // given ID field. Otherwise, it's not a valid Directory for snapshotting.
    private static final String SNAPSHOTS_ID = "$SNAPSHOTS_DOC$";

    // The index writer which maintains the snapshots metadata
    private final IndexWriter writer;

    /**
     * Reads the snapshots information from the given {@link Directory}. This
     * method can be used if the snapshots information is needed, however you
     * cannot instantiate the deletion policy (because e.g., some other process
     * keeps a lock on the snapshots directory).
     */
    public static Map<String, String> readSnapshotsInfo(Directory dir) throws IOException {
        IndexReader r = IndexReader.open(dir, true);
        Map<String, String> snapshots = new HashMap<String, String>();
        try {
            int numDocs = r.numDocs();
            // index is allowed to have exactly one document or 0.
            if (numDocs == 1) {
                Document doc = r.document(r.maxDoc() - 1);
                Field sid = doc.getField(SNAPSHOTS_ID);
                if (sid == null) {
                    throw new IllegalStateException("directory is not a valid snapshots store!");
                }
                doc.removeField(SNAPSHOTS_ID);
                for (Fieldable f : doc.getFields()) {
                    snapshots.put(f.name(), f.stringValue());
                }
            } else if (numDocs != 0) {
                throw new IllegalStateException("should be at most 1 document in the snapshots directory: " + numDocs);
            }
        } finally {
            r.close();
        }
        return snapshots;
    }

    /**
     * {@link PersistentSnapshotDeletionPolicy} wraps another
     * {@link IndexDeletionPolicy} to enable flexible snapshotting.
     * 
     * @param primary
     *          the {@link IndexDeletionPolicy} that is used on non-snapshotted
     *          commits. Snapshotted commits, by definition, are not deleted until
     *          explicitly released via {@link #release(String)}.
     * @param dir
     *          the {@link Directory} which will be used to persist the snapshots
     *          information.
     * @param mode
     *          specifies whether a new index should be created, deleting all
     *          existing snapshots information (immediately), or open an existing
     *          index, initializing the class with the snapshots information.
     * @param matchVersion
     *          specifies the {@link Version} that should be used when opening the
     *          IndexWriter.
     */
    public PersistentSnapshotDeletionPolicy(IndexDeletionPolicy primary, Directory dir, OpenMode mode, Version matchVersion) throws CorruptIndexException, LockObtainFailedException, IOException {
        super(primary, null);

        // Initialize the index writer over the snapshot directory.
        writer = new IndexWriter(dir, new IndexWriterConfig(matchVersion, null).setOpenMode(mode));
        if (mode != OpenMode.APPEND) {
            // IndexWriter no longer creates a first commit on an empty Directory. So
            // if we were asked to CREATE*, call commit() just to be sure. If the
            // index contains information and mode is CREATE_OR_APPEND, it's a no-op.
            writer.commit();
        }

        try {
            // Initializes the snapshots information. This code should basically run
            // only if mode != CREATE, but if it is, it's no harm as we only open the
            // reader once and immediately close it.
            for (Entry<String, String> e : readSnapshotsInfo(dir).entrySet()) {
                registerSnapshotInfo(e.getKey(), e.getValue(), null);
            }
        } catch (RuntimeException e) {
            writer.close(); // don't leave any open file handles
            throw e;
        } catch (IOException e) {
            writer.close(); // don't leave any open file handles
            throw e;
        }
    }

    @Override
    public synchronized void onInit(List<? extends IndexCommit> commits) throws IOException {
        // super.onInit() needs to be called first to ensure that initialization
        // behaves as expected. The superclass, SnapshotDeletionPolicy, ensures
        // that any snapshot IDs with empty IndexCommits are released. Since this 
        // happens, this class needs to persist these changes.
        super.onInit(commits);
        persistSnapshotInfos(null, null);
    }

    /**
     * Snapshots the last commit using the given ID. Once this method returns, the
     * snapshot information is persisted in the directory.
     * 
     * @see SnapshotDeletionPolicy#snapshot(String)
     */
    @Override
    public synchronized IndexCommit snapshot(String id) throws IOException {
        checkSnapshotted(id);
        if (SNAPSHOTS_ID.equals(id)) {
            throw new IllegalArgumentException(id + " is reserved and cannot be used as a snapshot id");
        }
        persistSnapshotInfos(id, lastCommit.getSegmentsFileName());
        return super.snapshot(id);
    }

    /**
     * Deletes a snapshotted commit by ID. Once this method returns, the snapshot
     * information is committed to the directory.
     * 
     * @see SnapshotDeletionPolicy#release(String)
     */
    @Override
    public synchronized void release(String id) throws IOException {
        super.release(id);
        persistSnapshotInfos(null, null);
    }

    /** Closes the index which writes the snapshots to the directory. */
    public void close() throws CorruptIndexException, IOException {
        writer.close();
    }

    /**
     * Persists all snapshots information. If the given id and segment are not
     * null, it persists their information as well.
     */
    private void persistSnapshotInfos(String id, String segment) throws IOException {
        writer.deleteAll();
        Document d = new Document();
        d.add(new Field(SNAPSHOTS_ID, "", Store.YES, Index.NO));
        for (Entry<String, String> e : super.getSnapshots().entrySet()) {
            d.add(new Field(e.getKey(), e.getValue(), Store.YES, Index.NO));
        }
        if (id != null) {
            d.add(new Field(id, segment, Store.YES, Index.NO));
        }
        writer.addDocument(d);
        writer.commit();
    }

}
